Once you have honed the chisel to 30 degree you will create burrs on flat side of the chisel. At this stage you will need to remove the burrs by rubbing the back of the chisel upnand down the oil stone/ Whet stone.Flattening the back isn't a case of trying to get the back of the chisel completely flat or removing scratches, let me explain.
The back of most flat chisels have a shallow dish ground into them at the manufacturing stage.
What you should see is an area about 10-15mm deep at the sharp end of the chisel that will be shiny, you may have a shiny area near the handle as well but the middle section will be dull this is where the factory shallow dish has been ground, the purpose of this dish is so you can flatten the point of the chisel, this process means you should end up with a very sharp chisel.